Low frequency acoustic and seismo-acoustic projectors find applications in marine seismic operations, underwater ocean acoustic tomography, long-range acoustic navigation and communications and deep-bottom penetration seismic profiling in the offshore oil and gas industry. Such sources may be used in Arctic under-ice acoustic far-range navigation and communications, underwater global positioning systems (RAFOS), and long-range ocean acoustic tomography and thermometry. Low-frequency underwater sound sources should be powerful and efficient.
The low frequency source can be an explosive (e.g. dynamite), or it can use more complicated technology such as an air gun providing single pulses, or vibroseis providing continuous frequency sweeps. Some acoustic sources in use for seismic applications, such as air gun, plasma (sparker) sound sources and boomers, are of the impulse type, where the transmitter emits a large non-coherent pressure pulse during a short time interval. Seismic air-gun surveys, such as those used in the exploration of oil and gas deposits underneath the ocean floor, produce loud, sharp impulses that propagate over large areas and increase noise levels substantially. Such a signal is not highly controllable, either in frequency content or repeatability. Coherent sound sources such as marine vibroseis may be much quieter and potentially less harmful for marine environments and should be used instead of air-guns in certain exploration activities.
Current continuous wave type sources make use of hydraulic, pneumatic, piezo-electric or magnetostrictive drivers and different type of resonance systems to store acoustic energy and to improve impedance matching, when generating low-frequency sound waves in water. The power output of a simple acoustic source is proportional to the squares of volume velocity and frequency and needs a large vibrating area to achieve reasonable levels. As a result, the sound source can become unacceptably large and expensive.
Seismic sources in the form of an underwater gas-filled balloon (or bubble) have been proposed and patented, for example in U.S. Pat. Nos. 8,441,892, 8,331,198, and 8,634,276, each of which is incorporated by reference herein in its entirety. A resonant bubble seismic source is a simple, efficient, narrow-band projector. The resonant bubble seismic source, also called a bubble resonator, may have a Q factor in shallow water that is approximately equal to 40 and its frequency band may be very narrow. The pressure gas-filled underwater bubble (or balloon from an elastic material) generates sound by a symmetrical air pump with open cylinders driven by an electrical motor. Seismic survey applications demand a large frequency band and underwater bubble sources may be mechanically tuned over a large frequency band.
To cover a large frequency band, a tunable air-bubble resonator has been patented, for example in U.S. Pat. No. 8,634,276. In that system, a projector changes its resonance frequency by mechanically changing the length of the air-duct between the two inside resonators. A computer-controlled, electromechanical actuator moves the cylindrical sleeve along the tube conducting air between two inside resonators, keeping the projector in resonance at the instantaneous frequency of a swept frequency signal. The computer synthesizes the linear frequency-modulated signal; compares the phase between transmitted and reference signals; and, using a Phase-Lock Loop (PLL) system, keeps the resonator bubble frequency in resonance with the driver frequency.
This tunable bubble seismic source works reasonably well at frequencies higher than 20 Hz, but at lower frequencies turbulent losses demand the large dimensions for a tunable air duct and for the whole resonator. Dimensions for a source with a frequency band of 5-20 Hz will be more than the maximum limit for a standard air-gun deployment system (4 tons). At the same time, there is a great interest and demand for a much lower frequencies down to 1 Hz. A high-Q tunable resonance systems have many other disadvantages: they are too sensitive to towing depth and water flow fluctuations; they have limitation on the frequency sweep rate; they transmit only specific waveforms with a slowly changing frequency; they are complicated and need a specially controlled system to keep the resonant frequency equal to the instant frequency of a transmitted signal; they have large start/stop transient time.